The present invention relates generally to an electronic control system for an automotive brake system, which controls hydraulic brake pressure in accordance with the load on the vehicle so as to prevent the vehicular wheels from locking. More specifically, the invention relates to an electronic automotive brake control system which controls the distribution of hydraulic brake pressure in a master cylinder of the brake system.
It should be noted that, throughout the disclosure, the word "load" means the physical weight on the vehicle due to passengers and/or luggage.
Conventionally, various kinds of load-dependent vehicular brake controls have been developed and proposed. One typical prior art system in this field is a linkage-type load-sensing proportioning valve disposed within a hydraulic circuit for rear wheel brakes. In brake control systems employing this linkage-type proportioning valve, the vertical displacement between the vehicle body, i.e. the sprung mass and wheel axles, i.e. the unsprung mass is detected to serve as a load-indicative parameter. The load value corresponding to the detected vertical displacement is applied to the proportioning valve to adjust the split point of the hydraulic braking pressure depending upon load on the vehicle.
This conventional brake control system can effectively adjust braking pressure as long as the suspension coil springs exhibit linear spring force characteristics so that the spring force increases linearly with vertical displacement between the sprung mass and unsprung mass. However, if the springs have non-linear spring force characteristics, it becomes difficult to detect the actual load distribution among the vehicular wheels, especially between the front and rear wheels. This tends to cause earlier locking of the rear wheels when the vehicular load is between the unloaded condition and fully-loaded condition.
This defect can be solved by adjusting the split point in the proportioning valve to match the non-linear spring characteristics of the suspension springs. However, in this case, different proportioning characteristics would be required for the proportioning valves for various types of vehicles employing different non-linear suspension springs. This prevents common use of the proportioning valves for different types of vehicles. This results in an unnecessarily expensive brake control system.